The goal of this project is to identify additional genes for inherited predisposition to breast cancer by studying families and patients of Ashkenazi Jewish (A J) ancestry. Several groups of people will be evaluated: ~60 AJ families with high incidence of breast cancer, ~900 unselected AJ breast cancer patients, ~100 AJ control families, and >3000 AJ population controls. The families and patients are wildtype by multiple tests at all known genes for inherited breast cancer. The proposed experimental and analytic approach is based on the distinctive historical demography of this 3opulation and combines linkage within families, haplotype sharing across families, resequencing candidate genes, and comparison of variant allele frequencies in independently ascertained cases vs. controls. This approach was validated by the identification of a previously undetected allele of CHEK2 co-segregating with breast cancer in a subset of these AJ families, then the demonstration that this allele was significantly more frequent among the AJ breast cancer cases than among AJ controls. Functionally, whereas wildtype human CHEK2 complemented the lethality of a Rad53 deletion in yeast, the mutant allele failed to do so. Most familial breast cancer remains unexplained by inherited mutations in BRCA1 or BRCA2 or other susceptibility genes. Discovery of additional breast cancer genes of moderate penetrance in the AJ population will be important to women both in this population and generally.